This invention relates to magnetic information storage and processing arrangements and more particularly to such arrangements in which stored binary information is represented in a magnetic medium as patterns of single-wall magnetic domains.
The advantcements in the single-wall magnetic domain technology in recent years have resulted in the realization of various and numerous data processing, memory, and other applications. The control of domain propagation in a thin film magnetic medium and the novel circuits achieved thereby are well-known and have been extensively treated in the general and patent literature. One mass memory arrangement, for example, is shown in the patent of P. I. Bonyhard et al., U.S. Pat. No. 3,701,132, issued Oct. 24, 1972, which memory is organized in the now familiar major-minor loop configuration where information is continuously circulated in a plurality of parallel, closed minor loops by an applied in-plane rotating magnetic field. A major loop at right angles and contiguous to the minor loops operates to accept information from the minor loops and acts as a temporary store to recirculate information transferred from the minor loops past a write-read position prior to restoration of the information (or replacement information) to vacancies in the minor loops created by the initial information transfer.
As mentioned, such a single-wall domain memory arrangement and others are well-known as in the manner in which the domains (commonly termed "bubbles") are initially created in the thin film medium. An external bias field of suitable polarity is applied perpendicularly to the plane of the medium to reduce randomly oriented, elongated domain patterns to the individual cylindrical, bubble-like domains usable as binary bit representations. The magnitude of the bias field largely controls the diameters of the bubbles created and as the bias field strength is increased, the diameter of a bubble is decreased until it ultimately collapses. Manifestly, it is important that the strength of the bias field be adjusted to ensure a stable bubble diameter, taking into consideration the properties of the material forming the magnetic medium. Were the bubbles to be permitted to increase or decrease in size during memory operation, serious domain interaction and variation in coupling to drive fields could result which, in turn, could lead to improper domain movement, as is also known.
Generally, in the past, a magnetic bias field value was selected to create an operating domain diameter which value lay substantially in the middle of a bias field range which corresponds to the stability range of domain diameters. The widest possible operating margins were thus ensured. The bias field was normally maintained at a constant level and the properties of the material of the thin film in which the domains were to be generated were prescribed to ensure a domain stability range which is constant as a function of temperature, for example, over a reasonable temperature range. As is known, materials having such properties were difficult to achieve, and in one prior art magnetic bubble arrangement, the bias field was adapted to vary as the stability range of the magnetic material varied with temperature. In the J. E. Geusic et al., U.S. Pat. No. 3,711,841, issued Jan. 16, 1973, for example, a bias field varying arrangement is described in which the bias field is supplied by a permanent magnet of a material having a temperature coefficient which causes the field to vary in a manner corresponding to the variations with temperature of the thin film stability range. As a result, the bias field, in tracking the stability range changes, remains within the operating bias range of the film in which the domains are moved. The reliability of the functional single-wall domain arrangement with which the permanent magnet bias field source is associated is thus enhanced.
The afore-mentioned Geusic et al., bias field varying arrangement offers one useful and advantageous means for contending with stability range changes in an environment where variations in steady state temperature may occur which affect equally the thin film layers and the permanent magnet biasing field source. Transient temperature changes may also cause changes in the stability range of a thin film magnetic layer and hence optimum domain diameter. Importantly, such transient temperature changes may not affect the biasing field magnet. Thus, for example, as a magnetic domain memory is assessed, power dissipation may vary and any such variation on the thin film layer will cause its temperature to be at variance with that of the biasing field magnet. This transient change would disappear if the memory usage were constant. In many memory applications, however, usage is not constant, and a continuing and varying temperature difference exists between the memory layers and the biasing magnet. Other factors may also affect the stability range of a given magnetic thin film. External mechanical stress, however slight, such as the flexing of a thin film substrate, for example, may cause serious stability range changes. Exposure to the presence of an external gross magnetic field could have similar detrimental effects.
It is, accordingly, one object of this invention to provide a new and novel, single-wall magnetic domain arrangement for tracking variations in the stability range of a thin magnetic film due to temperature changes and other causes.
Another object of this invention is the control of the biasing field of a single-wall magnetic domain arrangement in accordance with changes in the stability range of the thin film in which the single-wall domains are generated and propagated.
Still another object of this invention is a new and novel magnetic single-wall domain arrangement in which the biasing field is adjusted to achieve optimum domain diameters in view of changes in the thin film stability range.